Practical_Antenna_Handbook_0071639586

C h a p t e r 3 : A n t e n n a B a s i c s 87
Fields
Originally conceived as a way of helping us understand and predict the observed effect
of charges and magnets on other objects in the absence of any direct physical connection
between the source and the affected objects, fields have acquired a life of their own. This
“action at a distance” can be thought of as analogous to the way gravitational forces act.
If a voltage is applied between two points (in space, in a circuit, etc.), an electric field is
said to exist between those points. Similarly, if a current flows in a wire or other conducting
medium, a magnetic field is said to surround the conductor. A common high school
physics experiment provides graphic proof of this: As shown in Fig. 3.1, currents flowing
in the same direction in two parallel wires will cause the two wires to be attracted to each
other, and unrestrained sections of the two wires will actually move toward each other!
Reversing the direction of current flow in one of the wires will push the wires apart. The
force between the wires is proportional to the product of the magnitudes of the two currents
and inversely proportional to the distance between them. Note a very important
aspect of magnetic fields: The force is at right angles to the direction of current flow!
Whether electric or magnetic, however, all fields originate with electric charges. A
motionless electric charge creates a static, or non-time-varying, electric field. A moving electric
charge traveling at a constant velocity creates a static magnetic field (which is at right
angles to the direction of electron motion). Since a steady electric current in a wire is the
result of many electric charges moving with a constant speed past any point in that wire,
we can conclude that such a current will result in a steady magnetic field around the wire.
However, it is only when we rapidly vary the velocity of electric charges that electromagnetic
radiation (and, hence, radio waves) becomes possible. In the same way that you
cause your vehicle to go from being stopped at a traffic light to moving smoothly at 30
mph along a city street by accelerating it to the new speed after the light turns green, the
only way to cause the velocity of electric charges to vary is by accelerating or decelerating
those charges. In short, the possibility that radio waves can exist is the direct result of electric
charges undergoing
acceleration or deceleration.
While there is more
–
Battery
+
d
I 1
Force
I 2
– +
than one way to accelerate
charged particles, the
only method we need to
concern ourselves with
in this book is the application
of rapidly varying
voltages and currents to
wires or other conducting
structures.
Two other conditions
are necessary for gener-
Force I 1
x I 2
d
Battery
Figure 3.1 The force
between current-carrying
parallel wires.